Layers of nickel-phosphorous alloys deposited by electroless plating are frequently used as a barrier layer between a copper surface and a solderable or wire bondable material such as a palladium and/or gold layer in the manufacture of printed circuit boards, IC substrates and the like. The barrier layer prevents or retards undesired diffusion between the copper surface of a copper circuitry or a copper contact area and the noble metal layer and furthermore provides corrosion protection to said copper surface. Electroless deposition of nickel-phosphorous alloys onto rigid substrates is a mature technology.
A more recent application for nickel-phosphorous alloy layers is as barrier layers on flexible substrates in the manufacture of flexible printed circuit boards and the like. Such flexible substrates comprise a dielectric base material foil of a polymer material and a copper circuitry attached thereon. Parts of the copper circuitry designated as contact areas need to be coated with a nickel-phosphorous alloy layer which again serves as a barrier layer between the copper circuitry and noble metal layers such as palladium and/or gold layers deposited onto the barrier layer. Here, the bendability of the nickel-phosphorous alloy layer is mandatory for reliability of the flexible printed circuit board or related devices.
Plating baths for deposition of nickel-phosphorous alloys conventionally used in the manufacture of rigid printed circuit boards and the like leads to nickel-phosphorous alloy layers which are prone to undesired crack formation when bending the coated flexible substrate.
A plating bath for deposition of nickel-phosphorous alloys comprising nickel ions, hypophosphite ions and an aminocarboxylic acid is disclosed in JP 2006-206985A. The nickel-phosphorous layers obtained are bendable and suited for deposition and use on flexible substrates. This plating bath is free of carboxylic acids without an amino residue since otherwise no bendable nickel-phosphorous alloy layer can be deposited from said plating bath.
An electroless plating bath comprising nickel ions, a reducing agent, a complexing agent and a vertical growth inducer (bismuth ions) is disclosed in US 2010/0155108 A1. Nickel-phosphorous alloy layers deposited onto flexible substrates from said plating bath are sufficiently bendable and suited for application in flexible printed circuit boards and the like. However, toxic heavy metal ions are required in the plating bath for deposition of bendable nickel-phosphorous alloy layers.
An electroless plating bath for deposition of nickel phosphorous alloys comprising urotropine is disclosed in SU 272761. Urotropine is used as a stabilising agent which improves the shelf life of the plating bath compared to similar plating bath compositions comprising thiourea as the stabilising agent.
An electroless plating bath for deposition of nickel-phosphorous alloys onto aluminium and aluminium alloys is disclosed in U.S. Pat. No. 4,122,215. Formaldehyde can be added to the plating bath as a stabilising agent.